After silver halide material has been exposed and developed, it is necessary to remove the unexposed and hence undeveloped silver halide remaining in the material to render the material light stable. The most commonly used silver halide solvents used in fixing baths to remove or `fix-out` the undeveloped silver are either an alkali metal thiosulphate or ammonium thiosulphate. The thiosulphate first used was sodium thiosulphate, the hypo of the old photographic books. More recently, ammonium thiosulphate has been used as it removes the undeveloped silver very much faster than sodium thiosulphate. Potassium thiosulphate has also been proposed to be used, but as it is more expensive than sodium thiosulphate and not much if any faster at fixing out, it has not been used much commercially.
Most often the dissolved or fixed out silver is removed from the fixing solution by electrolysis, ion exchange or other means and the used desilvered fixing solution is discharged into the effluent system. However, for environmental reasons, in recent years the control of effluent has become very strict. Sodium or potassium salts which are very wide-spread in nature do not significantly affect the aqueous environment into which the effluent is discharged but ammonium salts tend to act as a fertilizer for the micro-organisms in the aqueous environment and if such micro-organisms proliferate greatly they reduce the oxygen content of the aqueous environment. This is in detriment to the higher species found in such environment such as fish. Further, when such microorganisms die, this further consumes oxygen and renders the water foul and unfit to support fish or higher-plant life. Thus it is desirable to find a fixing solution which contains no ammonium thiosulphate but which fixes out silver faster than sodium thiosulphate. In the past, various other salts have been used as fixing agents such as alkali metal thiocyanates but in general these are even less environmentally friendly than ammonium thiosulphate.